Cathode ray tube intensity compensation



March 20, 1956 w. T. SHREVE ET AL 2,739,264

CATHODE RAY TUBE INTENSITY COMPENSATION Filed Aug. 1, 1952 VOLTAGE INVEN TOR.

-WARRE/V 7.' SHREl E BY ROBE/FT J McCURD) I AHOIQya CATHODE RAY TUBEINTENSITY COIVHENSATION Warren '1. Shreve, Oreland, Pa., and Robert J.McCurdy, Bridgeboro, N. 5., assignors, by mesne assignments, to theUnited States of America as represented by the Secretary of the NavyApplication August 1, 1952, Serial No. 302,172

6 Claims. (Cl. 315-22) The invention described herein may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes without the payment of any royaltiesthereon or therefor.

This invention relates generally to cathode ray tube brightness controland more particularly to means for controlling the trace brightness inaccordance with average signal or beam deflection rate.

It has previously been proposed that the grid of the cathode ray tube bemodulated in accordance with the voltage applied for deflection of thebeam, this arrangement being particularly applicable to radar and othersimilar applications of the cathode ray tube. By this means the baseline is substantially eliminated and only the desired pips are shown onthe tube. It has also been proposed that the beam be intensified duringthe interval of deflection by an amount proportional to the average ofthe deflecting voltage in order that additional beam intensity beprovided to maintain trace brightness during the more rapid movement ofthe beam when it is being deflected. Various other modifications havebeen employed in an eifort to secure special effects or other featuresrelated to trace intensity.

In certain information display circuits employing a cathode ray tube ithas been the practice to employ a cathode voltage control for thepurpose of adjusting cathode ray beam intensity to provide appropriatevisibility. In such circuits the beam intensity may be selfregulating inthat a higher average beam intensity causes an increased cathode currentin the cathode ray tube and this increased current automaticallyelevates the cathode potential, as for example, when the cathode iscontrolled by a voltage drop across series-connected resistors between apositive voltage supply and ground, the cathode being connected to apoint in the series resistance array. The cathode ray tube control gridis then normally controlled by a separate voltage dropping device eitherbetween the positive supply and ground or between the positive supplyand a negative supply, such that the average grid potential can be heldat a desired potential either slightly above or slightly below ground inaccordance with cathode potential and required bias. In a circuit ofthis type the beam intensity is controlled in accordance with the beamintensity desired and may be adjusted by adjusting the potential atwhich the cathode is connected. This arrangement providesa means forlimiting brightness of beam trace but does not fully compensate forchanges in frequency of sweep or in duty cycle for a given sweepfrequency.

It is accordingly an object of the present invention to provide meansfor further controlling the beam intensity in accordance with theaveraged signal applied to the deflecting plates of the cathode raytubes.

A further object of the invention is to provide simple and efficientmeans for amplifying a fixed portion of the deflecting signal andapplying the amplified signal at a changed potential for control of beambrightness.

A still further object of the invention is to provide means fordecreasing average grid potential of a cathode ray tube as the sweepfrequency or sweep duration is increased.

A still further object is to provide simple means in a cathode ray gridcontrol circuit for transferring a beam intensity regulating voltage toa lower potential level.

Other objects and features of the invention will become apparent tothose skilled in the art as the disclosure is made in the followingdetailed description of a preferred embodiment of the invention asillustrated in the accompanying sheet of drawing in which:

Fig. 1 is a diagrammatic view of an electrical circuit in accordancewith present invention;

Fig. 2 illustrates a sweep voltage signal of two different frequenciesand two duty cycles, each with an averaged sweep voltage indicatedthereon.

Referring now to the drawings, wherein like reference charactersdesignate like or corresponding parts, there is shown in Fig. 1 acircuit for applying a portion of the sweep voltage, after averaging thesame, to an amplifying device and thence through voltage transferringmeans to a cathode follower tube from which an output signal is appliedin control of the grid potential of a cathode ray tube. This circuitcomprises 4 tubes of which 101 is preferably a triode connected as anamplifier, 102 and 103 are gaseous discharge tubes such as normally usedfor voltage regulators and 104 is preferably a triode arranged as acathode follower. The input for triode 101 is taken across resistor 11,one end of which is grounded and the input voltage thereacross isaveraged by an averaging circuit comprising series resistor 12 andparallel capacitor 21. The junction of 12 and 21 is connected to thegrid 31 of tube 101. Tube 101 may be self-biased by means of cathoderesistor 14 in any conventional manner or by a biasing battery 23between resistor 11 and ground, or by both means. Tube 101 is suppliedfrom a suitable direct current voltage source B+ of suitable value suchas 180 volts, the current supply being through resistor 13, theresistance of 13 being large with respect to the resistance of 14 inorder to provide appropriate amplification within the tube circuit.

Triode 104 is preferably connected at the plate thereof to the samepower source, the cathode being connected to ground through a loadresistor 16 in conventional cathode follower relationship. The grid 32of triode 104 is preferably controlled at a potential below the cathodepotential in accordance with the amplified signal from triode 101, thecircuit being provided by connection of two constant voltage dischargetubes 102 and 103 in series between the anode of 101 and the grid 32,connection therefrom being made through resistor 15 to a negativevoltage terminal, for example, volts negative. By this means it may beseen that voltage variations occurring at the relatively high positivepotential existing at the anode of amplifier 101 are transferredsubstantially without change through tubes 102 and 103 to the grid 32 atan appropriate lower potential. It is seen that tubes 102 and 103 arethus a potential dropping device interposed between series resistors 13and 15 connected between the positive and negative power terminals.

Since the averaging circuit comprising resistor 12 and condenser 21 isimperfect and the imperfections thereof become amplified in effect. intube 101 and resistor 13 a further condenser 22 is connected between theanode of tube 101 and ground in order to further stabilize the'positivevoltage at the anode of tubes 101 and 102.

The cathode ray tube 105 is shown schematically hav ing thereindeflecting electrodes 106 and 107, of which 107 is taken as the pair ofsweep producing electrodes energized by a suitable sweep generator whichproducw a sweep signal of variable duration and frequency, such PatentedMar. 20, 1956.

as 26. The accelerating electrode is conveniently connected to thepositive voltage supply employed for energization of tubes 101 and 104.The series resistors 17, 18 and 19 are also connected between the samepositive voltage supply and ground, connection being provided fromresistor 18 through a variablecontact arm to the cathode of tube 105.The accelerating voltage of the cathode ray tube accordingly is thevoltage between the positive voltage supply and the contact arm ofresistor 18 and remains fixed for any particular adjustment of beamintensity. if the grid 36 of tube 105 were connected to a fixed point inthe series resistor array 17, '18 and 19, the beam intensity wouldremain constant.

When the grid is connected to a separate voltage source generator 26 andground, and has an adjustable arm 25 thereon which is connected toresistors 11 and 12 and supplies the input voltage for amplifier 101-through the averaging circuit- Potential divider 24 is employed toselect any desired portion of the sweep voltage and apply the same as aninput to the brightness control circuit.

In Fig. 2 there is shown in a voltage against time diagram a typicalsweep voltage pattern a as employed in radar or other electronicdistance measuring apparatus. At b is shown a similar pattern atincreased frequency and at ca similar pattern except that the durationof the sweep has been increased to effect a longer duty cycle. Thedashed lines 33, 34 and 35 indicate average signal level for thepatterns at a, I) and c respectively. It is seen that the average signallevel in b and c is higher than in a. Therefore there is a rise involtage at grid 31 corresponding to the higher average sweep voltagewhenever the sweep freqeuncy or duty cycle is increased for a given tubepersistence and energization. Since either of these circumstances wouldresult in a brightened trace, by more frequent repetition and by longerdwell in each sweep of a given sweep velocity, an undesirablebrightening or fluctuation is encountered, even though some degenerationis inherent in the circuit whenever the cathode of tube 105 isresistively coupled to ground.

By the circuit means hereinbefore described an automatic compensation isprovided for screen brightness fluctuations due to causes such as changeof sweep frequency and change of duty cycle. The average signal to thesweep electrodes is taken in whole or in part, applied to the grid of anamplifier, the potential level of the amplified resultant change ofaverage sweep signal is transformed to a lower potential level, appliedto the coupling device, such as a follower triode circuit and thenapplied to the current control grid of the cathode ray tube in oppositepolarity to the change in average sweep voltage, thereby compensatingthe tendency of the trace to fluctuate. It is found that amplificationof the changes of averaged sweep voltage is generally necessary toeffect complete compensation. It is further desirable that the degree ofcompensation be readily adjustable and potential divider 24 providesthis adjustment while effectively coupling the sweep voltage to theaveraging circuit input.

Various modifications are contemplated and may obviously be resorted toby those skilled in the art without departing from the spirit and scopeof the invention, as

hereinafter defined by the appended claims, as only a preferredembodiment thereof has beendisclosed.

\Vhat is claimed is:

1. In an information display system including a cathode ray tube havingtherein a cathode, control grid and deflecting electrodes, thecombination of, a sweep generator adapted to produce a sweep signal ofvariable duration and frequency, said generator being connected toenergize the deflecting electrodes of the cathode ray tube, meansselecting a portion of the voltage energizing said electrodes, meansaveraging said selected voltage, means amplifying and inverting changesin said averaged selected voltage, non-linear means transferring saidamplified voltage from a higher to a lowerpotential level without changeof magnitude thereof, means coupling said transferred voltage to saidcontrol grid of the cathode ray tube, whereby changes in average voltageapplied to the electrodes are amplified, inverted and applied to preventcathode ray tube trace brightness changes.

2. In the system of claim 1 the transferring means comprising a gaseousdischarge means of the voltage regulator type connected between theoutput of the amplifying means and a potential point below the potentialof said cathode. i

3. In the system of claim 1 said coupling means comprising a cathodefollower circuit having a grid thereof at said lower potential level andcontrolled by said amplified voltage and having the output thereofconnected to said cathode ray tube grid.

4. In combination, a cathode ray tube having a cathode, a controltgridand deflecting means; a sweep generator adapted to producea sweep signalof variable sweep duration and sweep freqeuncy; an averaging circuit;means for applying the output of the sweep generator to the deflectingmeans of the cathode ray tube and for applying a portion of the outputof the generator to the averaging circuit; a'D. C. amplifying circuithaving an odd number of stages of amplification; a cathode followercircuit; a coupling circuit, said coupling circuit connecting the outputof the amplifying circuit to the input of the cathode follower circuit;the output of the cathode follower circuit being connected to thecontrol grid of the cathode ray tube; whereby, the light intensity ofthe cathode ray tube is of subsantially constant value irrespective ofthe length of each sweep or the magnitude of the sweep frequency of thesignal produced by the sweep generator.

5. In the combination defined in claim 4 in which the number of stagesof the D. C. amplification is one. 6. In the combination defined inclaim 5 in which the coupling circuit comprises a voltage regulatingtube and a resistor connected in series.

References Cited in the file of this patent UNITED STATES PATENTS2,313,967 Read Mar. 16, 1943 2,363,810 Schrader et al 'Nov. 28, 19442,399,754 Miller May 7, 1946 2,438,717 Puckette Mar. 30, 1948 2,448,299Dorne Aug. 31, 1948 2,453,711 Isbister Nov. 16, 1948 2,455,373 LesterDec. 7, 1948 2,465,364 Ferrar Mar. 29, 1949 2,569,240 King et al. Sept.25, 1951 2,620,455 Fockler Dec. 2, 1952 2,648,027 Geohegan Aug. 4, 1953

